The specific reductions, set for Jan. 1, will vary by market and station format.
The policy change reflects an inability of radio broadcasters to increase revenues, analysts said.
Arbitron, the widely respected ratings service, said the number of people who listen to radio remains high. But as commercial clutter has increased, the average time people actually listen each week has slid downward during the same period, to 19 hours and 30 minutes from 22 hours and 30 minutes.
Broadcasters have kept revenues relatively steady during decline by increasing the time devoted to commercials.
"If you have listened to the radio at all, you know that there is an amazing amount of commercial and promotional inventory," said John Hogan, head of Clear Channel Radio said.
"So much so that we have run the risk of diluting our product."